Work on wearable electronics has been ongoing for many years now, and in recent years some textile and wearable electronics devices have been introduced on the market. Examples include the Nike Fit, Adidas MiCoach, Hi-Call Bluetooth “Phone-Glove” and soon to be available Google Glass and the Apple Smartwatch. Conductive yarns and fabrics are commercially available, and can be coated or made entirely of metals or conductive carbons. However, textile energy storage and harvesting systems are still under development.
Energy harvesting systems include piezo-electric materials that produce electrical energy from body movements, wearable solar panels, thermoelectrics that could collect energy from body heat, or wireless Wi-Fi energy harvesting. Wireless harvesting poses advantages over other technologies, as it is ambient and does not require the wearer to be moving or specifically outside, and today most people are surrounded by Wi-Fi and broadband signals both at home and work. Thus, most people will be constantly charging their smart clothes.
Additionally, pairing these systems with energy storage (i.e., batteries or supercapacitors), means extra energy can be collected and stored for later use. A variety of combined energy harvesting and storage systems have been proposed, including tribo-electric systems with batteries as coin cells and flexible fibers that can act as both a solar cell and supercapacitor as illustrated in FIG. 1. FIG. 1 illustrates conventional hybrid energy storage and energy generation devices. FIG. 1(a) illustrates a fiber supercapacitor combined with a tribo-electric generator adapted to store and harvest energy from body movements. FIG. 1(b) illustrates a generator and supercapacitor that are powerful enough to light an LED. FIG. 1(c) illustrates a current/voltage curve of a single supercapacitor tested at 200 mV/s in a PVA-H3PO4 electrolyte. FIG. 1(d) illustrates a combined solar cell and pseudocapacitive fiber in liquid electrolytes. FIG. 1(e) illustrates a current/voltage curve of the supercapacitor tested at 0.5 V/s in 1M PVA-H3PO4 gel electrolyte and at different lengths. However, to date, no full fabric solutions with all of the integrated circuitry have been proposed.
The three main electrochemical energy storing technologies used in wearable systems (ranging from high power to high energy respectively) include electrical double layer capacitors (EDLCs), pseudocapacitors, and batteries. Both double layer and pseudocapacitors are commonly called “supercapacitors.” All of these devices typically consist of an electrode material, current collector, separator and electrolyte. FIG. 2 illustrates basic schematics for an a) all carbon EDLC (left), b) a pseudocapacitor (MnO2 depicted in center) and c) a lithium ion battery (right). All devices have an active material (e.g., carbon, MnO2, LiCoO2), a current collector, a separating membrane and an electrolyte (e.g., Na2SO4, or LiPF6 solutions). As shown in FIG. 2(a), EDLCs store charge in an electrostatic double layer between the surface of a charged electrode material and its respective counter ions. As shown in FIG. 2(c), batteries store charge through the conversion of chemical energy into electrical energy. Rechargeable secondary batteries use reactions that are reversible (e.g., lithium ion intercalation into graphite). As shown in FIG. 2(b), pseudocapacitors are devices that have both a double layer capacitance and fast surface redox or intercalation, which increases the energy density while maintaining fast charge and discharge times comparable to an EDLC.
Typical tests conducted to measure the capacitance and resistance in energy storage devices are cyclic voltammetry (CV), galvanostatic cycling (GC), and electrochemical impedance spectroscopy (EIS). Usually capacitance can be determined from CV and GC, and the equivalent series resistance (ESR) can be determined from GC and EIS.
Energy storing textiles can be categorized into 3 main groups: coated energy textiles, fiber and yarn electrodes, and custom woven and knitted textiles. Researchers began by coating pre-existing cotton or polyester textiles, either woven, knitted or non-woven, with various carbon or redox active electrode materials. Dip-coating, screen-printing, and painting were used to incorporate these materials into the fabric. However, multiple manufacturing challenges will need to be overcome for coated full fabrics as multiple layers of current collector, electrode, separator and encasement have to be incorporated into a single piece of fabric or a multi-layered garment.
The first reports of yarn or fiber-like supercapacitors and batteries came out between in 2011 and 2012. These planar materials could be transformed into 2-D and 3-D fabrics. From these reports, only a few groups report making their own woven or knitted textiles. The many reported textile supercapacitors which were tested at or around 0.2 A/g and 10 mV/s, the standard operating rates for conventional supercapacitors, are compared and contrasted below.
Capacitive fibers are the most promising materials for energy storing textiles because they can be knitted, woven or stitched into a fabric. If one knows the capacitance per length of the fiber/yarn, one can subsequently design a fabric with a specified total capacitance and resistance. Some examples of flexible energy storing fiber/yarn capacitors are shown in FIG. 3. The left column of FIG. 3 provides a schematic of a fiber yarn device, while the center column of FIG. 3 provides electrochemical data. The right column of FIG. 3 provides micrographs of real material. FIG. 3(a) illustrates a fiber supercapacitor encased in plastic tubing. FIG. 3(b) illustrates the resulting cyclic voltammogram tested at 1 V/s, while FIG. 3(c) illustrates an SEM image of the surface morphology of the graphite electrode material. FIG. 3(d) illustrates graphene fiber (GF) coated in polyelectrolyte. FIG. 3(e) illustrates the resulting cyclic voltammogram tested at 50 mV/s, while FIG. 3(f) illustrates an SEM image of the fiber cross section. FIG. 3(g) illustrates Biscrolled CNT-PEDOT fiber, and FIG. 3(h) illustrates the resulting cyclic voltammograms tested at 1V/s. FIG. 3(i) illustrates an SEM image of the fiber cross section, while FIG. 3(j) illustrates CNT coated cotton fiber with additional layers of Ppy and MnO2 encased in a plastic tube. FIG. 3(k) illustrates the resulting cyclic voltammograms, and FIG. 3(l) illustrates an SEM image of the fiber surface. FIG. 3(m) illustrates a schematic of a coaxial style lithium battery cable, while FIG. 3(n) illustrates the resulting charge-discharge curves, and FIG. 3(o) illustrates an optical micrograph of the cross-section.
A variety of textile supercapacitors have appeared in the scientific literature since 2009, including cotton or polyester textiles that have been coated in capacitive materials, fibers and yarns made entirely of capacitive materials, or full fabrics that incorporate all of the components of supercapacitors. However, the functionality of such devices is severely limited.
Also, with recent advancements in wireless communication, ultra-low-power electronics, and wearable technology, a new class of data networks has emerged for applications in which sensors are worn on the human body. A body sensor network (BSN), also known as a body area network (BAN), is a wireless system of low-power devices worn on or in the immediate proximity of the human body, capable of monitoring physiological functions or conditions in the surrounding environment. Body sensor networks have practical applications in a variety of industries including healthcare, entertainment, athletics, interactive gaming, consumer electronics, and the military. Body sensor networks (BSN) currently employ devices that are powered by battery sources, which pose a number of environmental and sustainability issues.
The field of body area networks evolved from technological advances in low-power integrated circuits and wireless communication, as well as a number of disadvantages presented by older technologies. For example, conventional electronics worn on the body are known to cause a great deal of discomfort to the user due to their rigidity and inability to conform to the contour of human anatomy. Additionally, many traditional biological sensors are powered using standard outlet and battery sources. Outlet power tethers the user and restricts movement, limiting the technology to mostly stationary applications. Battery sources present environmental issues due to waste created by their disposal.
In the healthcare industry, a preliminary study (published in November 2010) is being conducted at the Cardiology Unit of La Paz Hospital in Madrid, Spain to evaluate the combination of e-textiles and sensor devices for patient monitoring. This system utilizes two knit electrodes to measure bioelectric potential in the body, an accelerometer to measure patient movement, and thermometer to measure body temperature. The sensors and battery power source are enclosed in a case the size of a cassette tape. The battery occupies roughly 25% of this enclosure.
It is desired to develop an energy harvesting system on a textile substrate to wirelessly power body area network devices and eliminate the need for conventional batteries. Energy harvesting is a process in which energy collected from external sources, which can then be stored and converted into electrical energy. In radio-frequency applications, the source of harvested energy is electromagnetic radiation present in the ambient atmosphere or transmitted from an intentional radiator. If an intentional radiator is used, it must follow FCC regulations for maximum power radiated. Utilizing alternative energy sources will ensure that the system is sustainable and that operation will produce minimal negative impact on the environment as compared to solely battery powered devices.
It is particularly desired in accordance with the invention to develop an energy harvesting antenna and supercapacitor that are knitted within the same piece of fabric with little post production processing to produce electronic textiles that enable wireless and autonomous powering of body-worn sensors without the limitations of the prior art. The present invention addresses these and other needs in the art.